The proposed research project will be concerned on the biophysical properties of voltage-gated Ca channels in rat and frog skeletal muscle fibers. These studies are designed to define the physiological role of voltage-dependent Ca entry for muscle contraction. Previous studies (1 to 11) have described a voltage-dependent Ca entry in frog skeletal muscle fibers. These Ca channels are located in the tubular system and its role during muscle contraction is not clearly established. We plan to study, using the three microelectrode voltage clamp technique, the loose patch clamp technique and single channel recordings, the kinetic properties, permeation mechanisms, modulation and differentiation of the slow Ca channels. Moreover, the distribution of intracellular Ca during Ca current will be also investigated. In addition to this, some preliminary data indicate the existence of a fast Ca channel in muscle fibers. This preliminary finding will be carefully investigated in relation to the possible Ca entry during a single action potential or tetanic stimulation. An accurate description of the gating process, modulation and differentiation of Ca channels will be the basis to plan future mechanical experiments to determine the physiological role of voltage-dependent calcium entry.